A few scanning tips

www.scantips.com

Video Boards and Modes

The scanner creates 24 bit RGB images, and our video system should
match it to view those images.

Twenty five years ago (Windows 3.1 and early 486 vintage computers), video boards were
8 bit boards showing only 256 colors (indexed color, page 131).
Inferior today of course, but there were only a few scanners then.
Compuserve developed the GIF file format then, which was for the 8 bit images current then. However
Windows today can still select 256 color mode, even on the best video cards
today. Games often required it back then, and the Windows 9x installation default
was only 256 colors until we changed it. Windows 7 can still select 256
color mode now .

Our 24 bit scans will look pretty bad if the video board is set to
256 color mode (which also applies to images placed in a GIF file, which are only 256 colors). If your image results are poor (grainy, speckled,
mottled, splotchy), the first thing to check is your video mode (Windows
Control Panel - Display - Settings). It used to be a common problem
that people didn't realize they were stuck in 256 color mode,
causing any scanned images to look poor.

While you are at the video settings, be sure your screen is not stuck
in 640x480 pixel video size (Windows XP offers 640x480 only in
Compatibility mode now). 640x480 pixels is surely rather limiting for
scanning or even web browsing. 15 inch monitor screens are usually set to
800x600 pixels, and 17 inch screens are usually set to 1024x768 pixel
size.

In the mid 90's, we had 16 bit video boards (16 bit mode is
called High Color in Windows), and the modern era began, at least we
started seeing more photos on the screen then. The 16 bit High Color mode
is almost "good enough" quality to show photo images, at
least for most purposes. 16 bit color is 5 bits of each Red, Green and
Blue packed into one 16 bit word (2 bytes per pixel). 5 bits can show 32
shades of each primary RGB channel, and 32x32x32 is 32K colors. Green
used the extra one bit for 6 bits to achieve 64K colors overall, but half
of them are green. The human eye is most sensitive to green-yellow, and
more shades are a bigger advantage there. Green has twice the luminance
of Red, and six times more than Blue, so this is very reasonable. Video
boards do vary, but 24 bits is normally not so much better in most cases,
except in wide smooth gradients.

Video boards for the last few years are 24 bit color (called True
Color). Note that there is no 32 bit color. The confusion is
that 24 bit color mode normally uses 32 bit video mode
today, referring to the efficient 32 bit accelerator chips (word size).
The 24 bit color mode and so-called 32 bit video mode show the same 24
bit colors, the same 3 bytes RGB per pixel. 32 bit mode simply discards
one of the four bytes (wasting 25% of video memory), because having 3
bytes per pixel severely limits video acceleration functions.

Again the difference is this:
24 bit color in 24 bit video mode is three 8-bit bytes of RGB color information in three 8-bit bytes.
24 bit color in 32 bit video mode is three 8-bit bytes of RGB color information in one 32-bit memory word.

Processor chips can only copy data in byte multiples (8, 16, 32, or 64 bits). A 24
bit copy done with a hardware video accelerator would require three 8-bit
transfers per pixel instead of one 32-bit transfer. 32 bit video mode is
for speed, and it shows 24 bit color. There is no 32 bit "color".

24 bit color is 8 bits each of RGB, allowing 256 shades of each
primary color, and 256x256x256 = 16.7 million color combinations. Studies
show that the human eye can detect about 100 intensity steps (at any one
current brightness adaptation of the iris), so 256 tones of each primary
is more than enough. We won't see any difference between RGB
(90,200,90) and (90,201,90) but we can detect 1% steps (90,202,90) (on a
CRT tube, but 6-bit LCD panels show 1.5% steps). So our video systems
and printers simply don't need more than 24 bits.

If our screen is set to show say 1024x768 pixels, the video board has
memory chips to hold that current 1024x768 pixel screen image. This
memory image is the entire basis of our video system, it is what we see.
The video board requires 3 bytes of video memory per pixel for 24 bit
color, or 4 bytes for 32 bit video. So a 1024x768 pixel screen size (2.3
MB) requires a 4 MB video board. A 1280x1024 pixel screen size requires 4
MB if 24 bits, or 8 MB if 32 bit mode. Additional video board memory does
not affect 2D video speed, it only allows larger combinations of screen
size vs. color depth.

Memory is cheap today, and current video boards have 8 or 16 MB of
video memory (some have 128 MB for 3D games). We all probably have
sufficient video memory for most 2D screen sizes. But this is recent, it
certainly wasn't always that way, older computers (when memory was
expensive) had real shortcomings in that department not so long ago.

Some seeking faster video continue to use 16 bit color mode. They
should try 32 bit mode now. Even if 16 bit mode measures a bit faster,
today's computers are so fast that it is imperceptible to humans.
There's no reason to limit what we see now.

16 bit color allows far fewer tones than 24 bits. However, the actual
visual difference is not usually so different, 16 bits is often about
"good enough". Smooth gradients are most affected by 16 bit
color mode. 24 bit color has enough tones to show a smooth gradient like
the top image below. 16 bit color mode may see banding or streaks in
continuous colors, because the color is limited to 32 shades instead of
256 shades in each color. But 32 shades are a lot, and you won't
see many images like this anyway... perhaps in the skies of
landscapes.

To try to show that effect at worst case, here is a large
gradient fill made with the PhotoImpact Paint tool. 24 bit video boards
should show a smooth gradient. 16 bit mode will see numerous
vertical streaks where the display limits the colors to 32 shades instead
of 256 shades. Since a great width was necessary to show it, the fill was
768 pixels wide. The 32 shades in 16 bit mode should be about 768/32= 24
pixels wide each. On a 24 bit board, the 256 shades are about 768/256= 3
pixels wide. Still, 32 shades are quite a few, and frankly, it looks a
tad better in an image program than in a web browser. JPG compression
didn't seem to hurt it (this one is at 90% Quality), but here is a
TIF file (138K) of the same gradient image.

But if the above image looks all speckled and dotted like this partial
image below, then your video board is set to display only 256 colors, and
MUST BE CORRECTED if you have hopes of ever seeing images look their
best. This image below is the above file captured from a 256 color
640x480 video display. The speckled dots are smaller and harder to see at
higher screen resolutions, but I think you'll get the idea.

It is a bit naive to implicitly believe our own monitor is perfect
without testing it. Adjusting the monitor correctly is extremely
important to see images well. Many users cannot see the dark tones if it
is not bright enough, and everything dark appears full black. 24 bit
video boards should just be able to detect the end tones in the chart
below. 16 bit video mode will only see every other tone as unique, but you're doing OK if you see them.
Browsers do not have the best image quality reputations, I lose the first
black step when in Netscape, so this is better done in your favorite
image program.

Correct monitor adjustment is very important to how we see our images.
Many monitors (especially old ones) are not bright enough to distinguish
dark tones, and everything dark appears full black. Assume the Brightness
and Contrast controls are reversed (they really are). Show a large black
window (like a Dos or Cmd prompt window), and adjust the Brightness
control ¤ up just enough until the background Black
Level blooms to a detectable faint gray, and then back down just enough
to be a black again. Too low hides dark detail, too high reduces
contrast, so this Black Level is important. Then on regular brighter
screen content, set the Contrast control
for any pleasant
viewing brightness, and for good whites. It is probably most of the way
up.

If the monitor setting is available, set the monitor's color
temperature to 6500 degrees Kelvin, which is the standard for video, and
you will quickly get used to it. Then, some image editor programs offer a
monitor gamma adjustment in their Preferences menu (and unfortunately,
this is sometimes best left disabled, for example Paint Shop Pro and
Picture Publisher). This is not to be confused with Gamma, which is about
image data brightness at the midpoint, see page 168.

The purpose of a monitor gamma adjustment is to give a correct view of
images. The idea is to see it "right" so we can adjust it
"right", so our image is seen the same way on other monitors.
It calibrates video brightness at the midpoint. This does not affect the
image data at all, but only affects how it is seen in this program, which
may affect how you adjust the image.

The monitor gamma tool shows two areas of middle tone color, sometimes
gray, but often three RGB patches. One area is made of equal numbers of
pixels of 0 and 255 values, mixed 50% so that the averaged result is
assumed to be the correct middle tone, by definition. The Brightness and
Contrast settings determine these two end points. The other area is RBG
pixels of actual gray at mid-range value 128. This method compares three
points on the monitor's response curve; 0, 128, and 255. This
midpoint gain adjustment makes the two areas look alike in tone, bringing
midpoint 128 to the actual middle. When the gray tone appears the same
brightness as the mixed endpoint pixels, then the midpoint brightness is
considered correctly placed at 50%. This only affects images seen using
this one program, only for images viewed in that one program.